Photochemical water oxidation system using ruthenium catalysts embedded into vesicle membranes

• We construct a novel vesicular system for photocatalytic water oxidation.
• Ruthenium complexes embedded into vesicle membranes act as water oxidation catalysts.
• The efficiency for O2 production is larger than that in a similar homogeneous system.

Vesicles, approximately spherical bilayer membranes formed by self-organization of phospholipids, have been employed as reaction fields to construct artificial systems capable of converting light energy into chemical potential. To gain information about the performance of water oxidation catalysts in vesicular membranes, the complex [Ru(bda)(pyCO2Ocd)(pyCO2Me)] (H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid, pyR = pyridine having a substituent R at the 4-position, Ocd = (CH2)17CH3) was newly synthesized, and photochemical water oxidation using the complex embedded into vesicle membranes as a catalyst was investigated in an aqueous solution containing a sensitizer [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) and a sacrificial electron acceptor S2O82−. At first, the dependence of the initial rate of O2 production, vi, on the concentration of [Ru(bpy)3]2+, as well as S2O82−, was investigated to find optimum conditions to examine the performance of the catalyst. Subsequently, the dependence of vi on the catalyst concentration was examined in the vesicular system, indicating that vi increased approximately linearly with the catalyst concentration up to ∼30 μM, and declined after that. The turnover frequency for O2 production with respect to the catalyst was evaluated to be 39.1 h−1, which was larger than that in similar systems in a homogeneous solution by a factor of more than two. The reason for the linear dependence of vi on the catalyst concentration, as well as the superior performance of the catalyst in the vesicular system, was discussed on the basis of the mechanism accepted generally for the water oxidation catalyzed by related Ru complexes.

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